In adipocytes, insulin stimulates glucose transport primarily by promoting the translocation of GLUT4 to the plasma membrane. Requirements for Ca2+/ calmodulin during insulin-stimulated GLUT4 translocation have been demonstrated; however, the mechanism of action of Ca2+ in this process is unknown. Recently, myosin II, whose function in non-muscle cells is primarily regulated by phosphorylation of its regulatory light chain by the Ca2+/calmodulin-dependent myosin light chain kinase (MLCK), was implicated in insulin-stimulated GLUT4 translocation. The present studies in 3T3- F442A adipocytes demonstrate the novel finding that insulin significantly increases phosphorylation of the myosin II RLC in a Ca2+-dependent manner. In addition, ML-7, a selective inhibitor of MLCK, as well as inhibitors of myosin II, such as blebbistatin and 2,3-butanedione monoxime, block insulin- stimulated GLUT4 translocation and subsequent glucose transport. Our studies suggest that MLCK may be a regulatory target of Ca2+/calmodulin and may play an important role in insulin-stimulated glucose transport in adipocytes.
Protein Transport/drug effects ; Phosphorylation ; Naphthalenes/pharmacology ; Myosin-Light-Chain Kinase/antagonists & inhibitors/*metabolism ; Myosin Type II/*metabolism ; Mice ; Insulin/*pharmacology ; Glucose Transporter Type 4/*metabolism ; Enzyme Inhibitors/pharmacology ; Dose-Response Relationship, Drug ; Calmodulin/antagonists & inhibitors/physiology ; Azepines/pharmacology ; Animals ; Adipocytes/cytology/*drug effects/metabolism ; 3T3 Cells